The described subject matter relates to high bandgap phosphide-based III-V alloys for high efficiency optoelectronic devices by limiting intervalley carrier transfer.
One approach to achieving high efficiency white light emitting diodes (LEDs) is to combine individual red, green and blue LEDs (the so-called “RGB approach”). Such a device has a high color rendering index (CRT) for LED architecture, but requires that each of the individual LEDs also have high quantum efficiencies, defined as the ratio of emitted photons to electrons injected into the device. Both red and blue LEDs have already reached the necessary efficiencies, but green emission remains relatively inefficient. The desired green emission wavelength for a three-color mixing scheme is approximately 560 nm, which maximizes the CRI and relaxes the requirements for the red and blue emission as well. For a four color mixing scheme, an amber wavelength of approximately 575-590 nm may also be desired.
Historically, green-emitting LEDs have been based on the Ga1-xInxN and (AlxGa1-x)yIn1-yP alloys systems, in order to reach direct band gaps of Eg˜2.1-2.3 eV (at wavelengths λ˜540-590 nm). More recent efforts have also focused on using GaxIn1-xP alloys for this application as well. The nitride-based alloys are currently the only III-V alloy system suitable for short wavelength emission (λ<520 nm) since GaN has a direct bandgap in the UV (Eg=3.5 eV). The addition of In to GaN effectively translates the emission into the blue range, but further reduction of the gap into the green is accompanied by a severe reduction in emission efficiency. It is quite difficult to grow nitride-based semiconductors as freestanding substrates, therefore requiring the fabrication of Ga1-x InxN devices on foreign substrates that are not appropriately lattice-matched.
Conversely, (AlxGa1-x)yIn1-yP is lattice-matched in GaAs for y˜0.51, allowing for good material quality, and is the primary material system used for red and orange LEDs. However, the lattice-matched system is predicted to undergo a direct to indirect bandgap transition around 2.2-2.3 eV at approximately x=0.53, depending on the degree of spontaneous atomic ordering. Since photon emission is much less likely when the bandgap is indirect, (AlxGa1-x)0.51In0.49P cannot be used for LEDs operating at wavelengths below 560 nm. Furthermore, when considering that the bandgap must be several kT (˜100 meV) below the transition energy to prevent intervalley transfer of carriers to the X and L bands, which also lowers the emission efficiency, this alloy is capable of operating at high efficiency only up to the yellow-green edge of the spectrum (˜2.1 eV). GaxIn1-xP (no Al), that is slightly lattice-mismatched from GaAs, is also a candidate for green LEDs, but is also limited to similar wavelengths.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.